This application is based on and claims priority to Japanese Patent Applications No. 2000-194308, filed Jun. 28, 2000, and No. 2000-370872, filed Dec. 6, 2000, the entire contents of which are hereby expressly incorporated by reference.
1. Field of the Invention
The present invention generally relates to an exhaust system for an outboard motor, and more particularly relates to an improved exhaust system for an outboard motor that is powered by a multiple cylinder engine.
2. Description of Related Art
A typical outboard motor comprises a power head including an internal combustion engine and a housing unit depending from the power head. Recently, many outboard motors are powered by a multiple cylinder engine because of the better engine performance when compared to a single cylinder engine. The multiple cylinder engines for the outboard motors generally present a number of design obstacles. Some of the design obstacles are problems relating to configurations and arrangements of the exhaust system for the multiple cylinder engine. In outboard motor applications, unlike many other types of vehicle applications, space is very limited. The entire exhaust system for the outboard motor must be confined within the power head and the housing unit.
In some outboard motors, the exhaust system for the outboard motor is formed in substantial part by a cylinder block of the engine. That is, a single exhaust manifold, which communicates with exhaust ports, is formed with the cylinder block and the exhaust gases are delivered downwardly to an exhaust system in the housing unit. Due to the compact nature of the engine, the exhaust manifold is relatively short (compared to engine designs used in other applications, e.g., automotive) and hence the exhaust gases must merge together shortly downstream of the exhaust ports. A problem thus arises because the exhaust gases, coming from different exhaust ports interfere with each other. More specifically, the effect of pulse back from one exhaust port to another occurs. The effect can lessen the engine performance.
U.S. Pat. No. 5,806,311 discloses a solution to the problem. The arrangements for an outboard motor disclosed in this patent include a pair of exhaust manifolds allotted to two groups of cylinders. The cylinders served by the exhaust manifolds are fired so that no two cylinders served by the same exhaust manifold fire consecutively. Although the arrangements are useful for inhibiting the mutual interference of the exhaust gases coming from the different ports, the exhaust manifolds still are bulky and difficult to arrange in the limited space of the outboard motor.
A need therefore exists for an improved exhaust system for an outboard motor that can provide a construction that is compact enough for the space of an outboard motor despite having at least two exhaust manifolds.
It also is well known in four-cycle engine design to have open both the intake valve(s) and exhaust valve(s), which are associated with the same cylinder, for a period of time near Top Dead Center (TDC) as the piston completes the exhaust stroke and begins the intake stroke. The total angular movement of the crankshaft when both inlet and exhaust valves are simultaneously open in the TDC region is know as the overlap period. The reason for this overlap period is to induce as much fresh charge as possible into the cylinder during one combustion cycle (i.e., one four-stroke cycle).
The inlet valve opens toward the end of the exhaust stroke when the outgoing stream of exhaust gases in the exhaust port has sufficient velocity to form a depression in its wake (i.e., behind it in the exhaust port and combustion chamber). As a result, the fresh charge in the induction port will be drawn in the direction of the escaping exhaust gases, so that, in effect, it fills the combustion chamber space as it sweeps out the remaining exhaust gases.
The delay in closing the exhaust valve until after the piston begins the induction stroke also utilizes the partial vacuum created in the exhaust port and surrounding area of the combustion chamber by the exiting exhaust gases. This vacuum draws fresh charge into the combustion chamber as the inlet valve continues to open and the piston itself has not yet created a large vacuum pump effect.
Tuning the length of the exhaust pipe communicating with the exhaust port can increase the vacuum effect created during the overlap period. The length of the exhaust pipe will influence the timing of a pressure wave pulse reflected at the end of the pipe. The pressure-wave pulse desirably is timed so that the first reflected pressure wave reaches the port towards the beginning of the induction and the end of the exhaust period generally at its peak negative amplitude. The negative-pressure wave hitting the exhaust port during the overlap period helps extract (scavenge) the residual exhaust gases from the cylinder and induces the fresh charge to the enter the cylinder. This pulsation wave effect greatly improves air-charging efficiency.
Relatively long exhaust passages are necessary to obtain the pulsation wave effect. However, due to the noted shortage of space, it is difficult to achieve this effect in an outboard motor engine.
Another need thus exists for an improved exhaust system for an outboard motor that can produce the pulsation wave effect despite a limited space at least over some range of engine speeds and loads.
In accordance with one aspect of the present invention, an outboard motor comprises an internal combustion engine. A support member is arranged to support the engine. The engine includes a cylinder block defining a plurality of cylinder bores. The bores extend generally horizontally and are spaced apart vertically from each other to form a cylinder bank. The cylinder block further defines at least two exhaust manifolds that extend generally vertically along side the cylinder bank. Pistons reciprocate within the cylinder bores, and a cylinder head closes the ends of the cylinder bores to define combustion chambers together with the cylinder bores and the pistons. The cylinder head further defines a plurality of exhaust ports and a plurality of exhaust passages. Each combustion chamber has at least one exhaust port, and each exhaust passage communicates with a respective one of the combustion chambers through at least one of the exhaust ports. The exhaust passages are connected to the exhaust manifolds so that at least one of the exhaust passages is allotted to each one of the exhaust manifolds.
In accordance with another aspect of the present invention, an outboard motor comprises an internal combustion engine. A support member is arranged to support the engine. The engine includes a cylinder block, which defines a plurality of cylinder bores, and at least two exhaust manifolds. Pistons reciprocate within the cylinder bores, and a cylinder head closes the ends of the cylinder bores to define combustion chambers together with the cylinder bores and the pistons. The cylinder head further defines a plurality of exhaust ports and a plurality of exhaust passages. Each combustion chamber has at least one exhaust port, and each exhaust passage communicates with a respective one of the combustion chambers through at least one of the exhaust ports. The exhaust passages communicate with the exhaust manifolds so that at least one of the exhaust passages is connected to each exhaust manifold. At least one of the exhaust manifolds includes a downpipe section and a detour section that lies between one of the exhaust passages and the downpipe section.
In accordance with a further aspect of the present invention, an outboard motor comprises an internal combustion engine. A support member is arranged to support the engine. The engine includes a cylinder block defining a plurality of cylinder bores. The cylinder bores extend generally horizontally and are spaced apart from each other to form a cylinder bank. The cylinder block further defines at least two exhaust manifolds. Pistons reciprocate within the cylinder bores, and a cylinder head closes the ends of the cylinder bores to define combustion chambers together with the cylinder bores and the pistons. The cylinder head further defines a plurality of exhaust ports and a plurality of exhaust passages. Each combustion chamber has at least one exhaust port, and each exhaust passage communicates with a respective one of the combustion chambers through at least one of the exhaust ports. The exhaust passages are connected to the exhaust manifolds so that at least one of the exhaust passages is allotted to each one of the exhaust manifolds. The exhaust manifolds in turn join together at a location lower than the lower-most cylinder bore.
In accordance with a still further aspect of the present invention, an outboard motor comprises an internal combustion engine. A support member is arranged to support the engine. The engine includes a cylinder block defining a plurality of cylinder bores that are disposed in line to form a cylinder bank. The cylinder block further defines at least two exhaust manifolds that extend aside the cylinder bank. Pistons reciprocate within the cylinder bores, and a cylinder head closes the ends of the cylinder bores to define combustion chambers together with the cylinder bores and the pistons. The cylinder head further defines a plurality of exhaust ports and a plurality of exhaust passages. Each combustion chamber has at least one exhaust port, and each exhaust passage communicates with a respective one of the combustion chambers through at least one of the exhaust ports. The exhaust passages communicate with the exhaust manifolds so that at least one of the exhaust passages is connected to each exhaust manifold. A crankshaft is coupled to the pistons and is journaled for rotation about a crankshaft axis. The cylinder bores and the crankshaft are arranged such that a first plane, which contains the crankshaft axis, lies parallel to and offset from a second plane, which contains axes of the cylinder bores. The first plane is offset to a side of the second plane on which the exhaust manifolds are disposed.
In accordance with a yet further aspect of the present invention, an outboard motor comprises an internal combustion engine. A support member is arranged to support the engine. The engine includes a cylinder block defining a plurality of cylinder bores and at least two exhaust manifolds. The engine includes a cylinder block defining a plurality of cylinder bores and at least two exhaust manifolds. The cylinder bores and the exhaust manifolds have end openings that face generally in the same direction. Pistons reciprocate within the cylinder bores, and a cylinder head closes the end openings of the cylinder bores to define combustion chambers together with the cylinder bores and the pistons. The cylinder head further defines at least one exhaust port per combustion chambers and exhaust passages that communicate with the exhaust ports. The exhaust passages are coupled with the exhaust manifolds at the end openings of the exhaust manifolds so that at least one of the exhaust passages is allotted to each one of the exhaust manifolds.